solfege: add missing build inputs

Fixes these ./configure messages:

  $ nix-build -A solfege | grep " no$"
  [... checking whether we are cross compiling... no ...]
  checking for swig... no
  checking for lilypond... no
  checking for gs... no
  checking for xml2po... no
  checking for xsltproc... no
  checking for /usr/share/sgml/docbook/stylesheet/xsl/nwalsh/html/chunk.xsl... no

pkgs/by-name/so/solfege/package.nix

@@ -4,16 +4,21 @@
   autoconf,
   automake,
   csound,
+  docbook-xsl-ns,
   fetchurl,
   gdk-pixbuf,
   gettext,
+  ghostscript,
+  gnome-doc-utils,
   gobject-introspection,
   gtk3,
   librsvg,
+  libxslt,
   lilypond,
   mpg123,
   pkg-config,
   python3Packages,
+  swig,
   texinfo,
   timidity,
   txt2man,
@@ -46,10 +51,16 @@ python3Packages.buildPythonApplication rec {
   nativeBuildInputs = [
     autoconf
     automake
+    docbook-xsl-ns
     gdk-pixbuf
     gettext
+    ghostscript
+    gnome-doc-utils
     gobject-introspection
+    libxslt
+    lilypond
     pkg-config
+    swig
     texinfo
     txt2man
     wrapGAppsHook3
@@ -65,6 +76,10 @@ python3Packages.buildPythonApplication rec {
     pygobject3
   ];
 
+  configureFlags = [
+    "--enable-docbook-stylesheet=${docbook-xsl-ns}/share/xml/docbook-xsl-ns/html/chunk.xsl"
+  ];
+
   preBuild = ''
     sed -i -e 's|wav_player=.*|wav_player=${alsa-utils}/bin/aplay|' \
            -e 's|midi_player=.*|midi_player=${timidity}/bin/timidity|' \